1. Field of the Invention
The present invention relates to an image transferring device for an image forming apparatus such as a copier, printer, facsimile transceiver or similar photographic image forming apparatus in which an image is electrostatically formed on an image carrier. More particularly, the invention is concerned with an image transferring device for transferring an image from the image carrier to a transfer belt while transporting a transfer sheet and while the transfer sheet is electrostatically adhered to the transfer belt.
2. Description of the Related Art
Japanese Patent Laid-Open Publication No. 3-167579 discloses a conventional image transferring device for an image forming apparatus, with the transfer belt device provided for an image forming apparatus such as a copier, or a printer.
Referring to FIG. 8, with such a transfer belt device for the image forming apparatus, a transfer belt 1 is disposed below a photosensitive drum 7 and passes over a conductive drive roller 2 and a conductive driven roller 3. The conductive drive roller 2 is connected to a motor, not shown, and is rotated in a direction indicated by an arrow in the figure. As the conductive drive roller 2 is rotated, the transfer belt 1 is moved in a direction for transferring a transfer sheet 6 (indicated by the arrow in the figure).
A bias roller 4 is located downstream of the conductive driven roller 3 with respect to the moving direction of the transfer belt 1. The bias roller 4 is held in contact with an inner surface of the transfer belt 1. A power source 5 is connected to the bias roller 4 and applies to the transfer belt 1 a charge which is opposite in polarity to that of the toner deposited on the photosensitive drum 7. The conductive drive roller 2 is connected to ground so as to allow a flow of electric current from the transfer belt 1 to ground. The electric current is fed to the transfer belt 1 via the bias roller 4 from the power source 5. An eraser, not shown, is disposed near the conductive driven roller 3 so as to remove the charge from the transfer belt 1 by irradiation.
The transfer sheet 6 is delivered from a paper feeding device, not shown. The transfer sheet 6 is polarized by charging, in which the charge is applied from the bias roller 4 via the transfer belt 1.
An electrostatic charge is generated on the basis of the relation between a net charge on the transfer belt 1 and a polarized charge on the transfer sheet 6. The transfer sheet 6 is thus adhered onto the transfer belt 1 by the electrostatic charge. A toner image is transferred from the photosensitive drum 7 to the transfer sheet 6, and the transfer sheet 6 on which the toner image is formed is delivered by the transfer belt 1. The transfer sheet 6 is then separated from the transfer belt 1 at the location of the conductive drive roller 2 by the rigidity of the transfer sheet 6, which is also known as a curvature separation (i.e., as the sheet passes over the curvature of the roller).
However, in the above-described conventional transfer belt device, when the power source 5 supplies the electric current to the transfer belt 1 via the bias roller 4, a surface potential of the conductive driven roller 3 is substantially equal to a surface potential of the bias roller 4. The photosensitive drum 7 is held in contact with the transfer belt 1 to form a nip portion 8, and the toner image is normally transferred from the photosensitive drum 7 to the transfer sheet 6 at the nip portion 8. However, in this condition, the toner image is transferred from the photosensitive drum 7 to the transfer sheet 6 upstream of the nip portion 8 with respect to the moving direction of the transfer belt 1, which is called a pre-transfer.
As a result of the pre-transferring of toner, the toner image of the photosensitive drum 7 is not transferred to a correct position of the transfer sheet 6 and thus the quality of an image on the transfer sheet 6 is degraded.
In addition, dust can be transferred to the transfer sheet 6 under the condition of the pre-transfer, thereby further degrading the quality of an image on the transfer sheet 6.
Further, the conductive drive roller 2 has a rubber surface so as to prevent slippage between the transfer belt 1 and the conductive drive roller 2, with the rubber of the conductive drive roller 2 formed of a conductive material. However the cost of the rubber made of a conductive material is expensive, and thus the production cost of the above-mentioned transfer belt device is increased.